ixgbe: fix unbalanced device enable/disable in suspend/resume

In the Linux kernel, the following vulnerability has been resolved:

ixgbe: fix unbalanced device enable/disable in suspend/resume

pci_disable_device() called in __ixgbe_shutdown() decreases
dev->enable_cnt by 1. pci_enable_device_mem() which increases
dev->enable_cnt by 1, was removed from ixgbe_resume() in commit
6f82b2558735 ("ixgbe: use generic power management"). This caused
unbalanced increase/decrease. So add pci_enable_device_mem() back.

Fix the following call trace.

ixgbe 0000:17:00.1: disabling already-disabled device
Call Trace:
__ixgbe_shutdown+0x10a/0x1e0 [ixgbe]
ixgbe_suspend+0x32/0x70 [ixgbe]
pci_pm_suspend+0x87/0x160
? pci_pm_freeze+0xd0/0xd0
dpm_run_callback+0x42/0x170
__device_suspend+0x114/0x460
async_suspend+0x1f/0xa0
async_run_entry_fn+0x3c/0xf0
process_one_work+0x1dd/0x410
worker_thread+0x34/0x3f0
? cancel_delayed_work+0x90/0x90
kthread+0x14c/0x170
? kthread_park+0x90/0x90
ret_from_fork+0x1f/0x30